Properties of WCAg and WCCu for vacuum contactors

This paper compares the switching properties of WCAg and WCCu for vacuum contactors and studies the influence of material composition and grain size of the WC powder. Different materials are compared with respect to their breaking capability, the state of their contact surfaces after arcing, their erosion losses, and their chopping current. The experiments were carried out with a demountable vacuum chamber in a synthetic test circuit. For the switching capability tests, the contacts were stressed by arc currents up to 7 kA root-mean square (rms), and a transient recovery voltage of 23 kV peak. The chopping measurements were carried out at a test current of 45-A rms. For all materials investigated there was an adverse ranking between interruption capability on the one hand and chopping current on the other. Materials with a higher WC content showed lower chopping currents, but also lower interruption limits. A general explanation is that all factors keeping the arc stable down to lower currents in the case of chopping conditions, also favor re-establishment of the arc at current zero on high current stress. Furthermore, materials with lower interruption limits exhibit stronger cracks and spikes.